Most of the deep space telecommunications links use circular polarization. One reason for this is because of the operational simplicity of tracking a circularly polarized signal as opposed to a linearly polarized signal. However, linear polarization provides an opportunity to perform scientific investigations and engineering tasks that cannot be done with circular polarization. This being the case, for some space missions it would be desirable to have a downlink from the spacecraft that is linearly polarized. Since the Deep Space Network (DSN) normally radiates a circularly polarized uplink, it has been recognized that a penalty for providing a linear downlink would be 3 dB loss in the uplink due to polarization mismatch. An implementation that provides a linear downlink capability without suffering the 3 dB uplink loss is desirable.
The linearly polarized downlink on the spacecraft would provide an opportunity to conduct investigations, such as of the solar corona with Faraday rotation techniques. In addition, a linearly polarized downlink would provide an opportunity for calibrating the Earth's ionosphere. The calibration of the ionosphere is needed by some deep space missisons to provide precision navigation. Present ionospheric calibration methods are problematic (not line of sight) and use linearly polarized VHF satellites which are becoming less available. Use of a linearly polarized signal in the line-of-sight would be an attractive alternative, that is, use the same downlink for telemetry, ionospheric calibrations, and scientific investigations.